Jupiter's comet

Jupiter's comet impact leaves
mystery

By Robert Roy Britt
SPACE.com

(SPACE.com) -- Jupiter's atmosphere still contains remnants of
a comet impact from a decade ago, but scientists said last week
they are puzzled by how two substances have spread into different
locations.

The new study also discovered two previously undetected chemicals
in Jupiter's air.

Grasping what chemical compounds are in and above the Jovian clouds
and how they move about could help scientists understand planets
outside our solar system, too, said the researchers who produced
the work.

From July 16 through July 22, 1994, more than 20 fragments of
Comet P/Shoemaker-Levy 9 collided with the gaseous planet, all
coming in at about the same latitude, 45 degrees south. Fragments
up to 1.2 miles (2 kilometers) sent plumes of hot gas into the
Jovian atmosphere. Dark scars lasted for weeks.

Shocks created by the impacts led to high-temperature chemical
reactions that produced hydrogen cyanide, which remains in the
air but has been spread around a bit in the years since. The comet
also delivered carbon monoxide and water, which through an interaction
with sunlight, scientists suspect, was converted to carbon dioxide.

The Cassini spacecraft, now at Saturn, examined Jupiter as it
swung by. The new study draws on infrared data from Cassini collected
in 2000 and 2001.

The hydrogen cyanide has diffused some both north and south, mixed
by wave activity, explained Michael Flasar of NASA's Goddard Space
Flight Center. Jupiter's cloud bands carry material around the
planet swiftly, but the bands do not mix easily. Not surprisingly,
hydrogen cyanide is most abundant in a belt at the latitude where
the comet was absorbed. At five degrees of latitude change in both
directions, its presence drops off sharply.

The highest concentration of carbon dioxide, however, has shifted
away from the latitude of the impact. It is most prevalent poleward
of 60 degrees south and decreases abruptly, toward the equator,
north of 50 degrees south. Another smaller spike in its presence
occurs at high northern latitudes, around 70 to 90 degrees north.

Perhaps the two chemicals got distributed at different altitudes,
and are being moved around by different currents, Flasar said.
Or maybe the formation of the carbon dioxide was more complex than
thought. He said it might have involved carbon monoxide first moving
away from the impact area and then interacting with other substances
at higher latitudes before being converted to carbon dioxide.

If all that sounds complicated, you're not alone in wondering
what's going on.

"We're scratching our heads, and we need to work through
these, and perhaps other, scenarios," said Flasar, who is
principal investigator for Cassini's Composite Infrared Spectrometer.

The study, led by Virgil G. Kunde of the University of Maryland,
was published Thursday in the online version of the journal Science.

The work also uncovered two new compounds, diacetylene and a so-called
methyl radical, which are products of the breakup of methane by
ultraviolet radiation from the sun. These were expected but had
not been observed at Jupiter before.

So far as astronomers know, the more than 100 giant planets found
outside our solar system might be something like Jupiter. Only
one has had its atmosphere probed. Better knowledge of the substances
in Jupiter, and how things move around, should help set the stage
for grasping the formation and evolution of gaseous extrasolar
planets, the researchers say.

"An understanding of the processes governing the composition
and distribution of chemical species in Jupiter's atmosphere is
required to successfully understand the chemical composition of
extrasolar planets," they write in the journal.